Slight change and add comment to layout assignation algo

src/rpc/layout.rs

@@ -175,47 +175,32 @@ impl ClusterLayout {
 		// Get old partition assignation
 		let old_partitions = self.parse_assignation_data();
 
-		// Create new partition assignation starting from old one
+		// Start new partition assignation with nodes from old assignation where it is relevant
-		let mut partitions = old_partitions.clone();
+		let mut partitions = old_partitions
+			.iter()
+			.map(|old_part| {
+				let mut new_part = PartitionAss::new();
+				for node in old_part.nodes.iter() {
+					if let Some(role) = node.1 {
+						if role.capacity.is_some() {
+							new_part.add(None, n_zones, node.0, role);
+						}
+					}
+				}
+				new_part
+			})
+			.collect::<Vec<_>>();
 
-		// Cleanup steps in new partition assignation:
+		// In various cases, not enough nodes will have been added for all partitions
-		let min_keep_nodes_per_part = (self.replication_factor + 1) / 2;
+		// in the step above (e.g. due to node removals, or new zones being added).
-		for part in partitions.iter_mut() {
+		// Here we add more nodes to make a complete (but sub-optimal) assignation,
-			// - remove from assignation nodes that don't have a role in the layout anymore
-			part.nodes
-				.retain(|(_, info)| info.map(|x| x.capacity.is_some()).unwrap_or(false));
-
-			// - remove from assignation some nodes that are in the same datacenter
-			//   if we can, so that the later steps can ensure datacenter variety
-			//   as much as possible (but still under the constraint that each partition
-			//   should not move from at least a certain number of nodes that is
-			//   min_keep_nodes_per_part)
-			'rmloop: while part.nodes.len() > min_keep_nodes_per_part {
-				let mut zns_c = HashMap::<&str, usize>::new();
-				for (_id, info) in part.nodes.iter() {
-					*zns_c.entry(info.unwrap().zone.as_str()).or_insert(0) += 1;
-				}
-				for i in 0..part.nodes.len() {
-					if zns_c[part.nodes[i].1.unwrap().zone.as_str()] > 1 {
-						part.nodes.remove(i);
-						continue 'rmloop;
-					}
-				}
-
-				break;
-			}
-		}
-
-		// When nodes are removed, or when bootstraping an assignation from
-		// scratch for a new cluster, the old partitions will have holes (or be empty).
-		// Here we add more nodes to make a complete (sub-optimal) assignation,
 		// using an initial partition assignation that is calculated using the multi-dc maglev trick
 		match self.initial_partition_assignation() {
 			Some(initial_partitions) => {
 				for (part, ipart) in partitions.iter_mut().zip(initial_partitions.iter()) {
 					for (id, info) in ipart.nodes.iter() {
 						if part.nodes.len() < self.replication_factor {
-							part.add(part.nodes.len() + 1, n_zones, id, info.unwrap());
+							part.add(None, n_zones, id, info.unwrap());
 						}
 					}
 					assert!(part.nodes.len() == self.replication_factor);
@@ -287,7 +272,7 @@ impl ClusterLayout {
 						let mut newpart = part.clone();
 
 						newpart.nodes.remove(irm);
-						if !newpart.add(newpart.nodes.len() + 1, n_zones, idadd, infoadd) {
+						if !newpart.add(None, n_zones, idadd, infoadd) {
 							continue;
 						}
 						assert!(newpart.nodes.len() == self.replication_factor);
@@ -422,7 +407,7 @@ impl ClusterLayout {
 							continue;
 						}
 						for (pos2, &qv) in q.iter().enumerate().skip(*pos) {
-							if partitions[qv].add(rep + 1, n_zones, node_id, node_info) {
+							if partitions[qv].add(Some(rep + 1), n_zones, node_id, node_info) {
 								remaining -= 1;
 								*pos = pos2 + 1;
 								break;
@@ -579,16 +564,27 @@ impl<'a> PartitionAss<'a> {
 		}
 	}
 
+	// add is a key function in creating a PartitionAss, i.e. the list of nodes
+	// to which a partition is assigned. It tries to add a certain node id to the
+	// assignation, but checks that doing so is compatible with the NECESSARY
+	// condition that the partition assignation must be dispersed over different
+	// zones (datacenters) if enough zones exist. This is why it takes a n_zones
+	// parameter, which is the total number of zones that have existing nodes:
+	// if nodes in the assignation already cover all n_zones zones, then any node
+	// that is not yet in the assignation can be added. Otherwise, only nodes
+	// that are in a new zone can be added.
 	fn add(
 		&mut self,
-		target_len: usize,
+		target_len: Option<usize>,
 		n_zones: usize,
 		node: &'a Uuid,
 		role: &'a NodeRole,
 	) -> bool {
-		if self.nodes.len() != target_len - 1 {
+		if let Some(tl) = target_len {
+			if self.nodes.len() != tl - 1 {
 				return false;
 			}
+		}
 
 		let p_zns = self
 			.nodes